1. Field of the Invention
This invention is concerned with a process for the manufacture of lubricating oils. In particular, it is concerned with a particular combination of unit processes whereby a hydrocracked lube oil of good stability and low pour point is produced with high energy efficiency.
2. Prior Art
Refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become a highly developed and complex art. Although the broad principles involved in refining are qualitatively understood, the art is encumbered by quantitative uncertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular constitution of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 450.degree. F., the molecular weight of the hydrocarbon constituents is high and these constituents display almost all conceivable structures and structure types. This complexity and its consequences are referred to in "Petroleum Refinery Engineering," by W. L. Nelson, McGraw Hill Book Company, Inc., New York, N.Y., 1958 (Fourth Edition), relevant portions of this text being incorporated herein by reference for background.
In general, the basic notion in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which basically are physical separation processes in the sense that if all the separated fractions were recombined one would reconstitute the crude oil.
Unfortunately, crude oils suitable for the manufacture of lubes are becoming less available due to exhaustion of reserves, and the reliability of a steady, adequate supply from a known source is a matter of concern due to political instability.
The desirability of upgrading a crude fraction normally considered unsuitable for lubricant manufacture to one from which good yields of lubes can be obtained has long been recognized. The so-called "hydrocracking process," sometimes referred to in the art as "severe hydrotreating," has been proposed to accomplish such upgrading. In this process, a suitable fraction of a poor grade crude, such as a California crude, is catalytically reacted with hydrogen under pressure. The process is complex in that some of the oil is reduced in molecular weight and made unsuitable for lubes, but concurrently a substantial fraction of the polynuclear aromatics is hydrogenated and cracked to form naphthenes and paraffins. Process conditions and choice of catalyst are selected to provide an optimal conversion of the polynuclear aromatic content of the stock, since this component degrades the viscosity index and stability of the stock. Also, in the hydrocracking process, paraffins can be isomerized, imparting good V.I. characteristics to the final lube product. For purposes of this invention, the term "hydrocracking" will be employed for the foregoing process step and to distinguish this step from the "hydrotreating" step to be described below, the purpose of the latter being to stabilize the lube base stock produced by hydrocracking. For purposes of this invention, the hydrocracking and hydrotreating steps may be distinguished also by the amount of hydrogen consumed, the hydrocracking step typically consuming about 1000-2000 SCF/bbl (standard cubic feet per barrel of feed) while the hydrotreating step consumes only about 100-200 SCF/bbl.
The hydrocracking process for increasing the availability of lube oils has an attractive feature that is not immediately apparent. Generally, the composition and properties of hydrocracked stocks are not particularly affected by the source and nature of the crude, i.e. they tend to be much more alike than lube fractions prepared from different crudes by conventional means. Thus, the process promises to free the refiner from dependence on a particular crude, with all of the advantages that this freedom implies.
Hydrocracked lube stocks, however, tend to be unstable in the presence of air when exposed to sunlight. On such exposure, a sludge is formed, sometimes very rapidly and in fairly substantial amount. This tendency in a lubricating oil is unacceptable. Additionally, some hydrocracked lube oils tend to darken or to form a haze.
Several methods have been proposed to correct the above-described instability. U.S. Pat. No. 4,031,016 to Berger et al proposes to add certain antioxidants to the hydrocracked oil. A second proposed approach is to hydrotreat the hydrocrackate. Variants of this approach are described in U.S. Pat. No. 3,666,657 which utilizes a sulfided mixture of an iron group metal and a Group VI metal for the hydrotreating stage; U.S. Pat. No. 3,530,061 which utilizes a hydrotreating catalyst having one or more elements from Group IIB, VIB and VIII at hydrogen pressure up to about 100 psig; and U.S. Pat. No. 4,162,962 which teaches to hydrotreat the hydrocrackate at a temperature in the 200.degree. to 300.degree. C. range with a catalyst of prescribed pore size. U.S. Pat. No. 3,530,061 to Orkin et al utilizes a non-cracking support for the hydrotreating stage. U.S. Pat. No. 3,852,207 to Stangeland et al teaches to hydrotreat with a noble metal hydrogenation component supported on an oxide. The patents cited above are believed representative of the state of the art, and each is incorporated herein by reference.
Hydrocracked lubricating oils generally have an unacceptably high pour point and require dewaxing. Solvent dewaxing is a well-known and effective process but expensive. More recently catalytic methods for dewaxing have been proposed. U.S. Pat. No. Re. 28,398 to Chen et al describes a catalytic dewaxing process wherein a particular crystalline zeolite is used. To obtain lubricants and specialty oils with outstanding resistance to oxidation, it is often necessary to hydrotreat the oil after catalytic dewaxing, as illustrated by U.S. Pat. No. 4,137,148 to Gillespie et al. The foregoing patents are incorporated herein by reference as background, and indicate the state of the dewaxing art.
It is inferentially evident from the foregoing background material that the manufacture of modern high quality lubricants in general requires that the crude be treated in a sequence of fairly complex and costly steps. It is further evident that there is a need for processes which can efficiently provide such lubricants from interchangeable and readily available low grade crudes.
It is an object of the present invention to provide an improved process for the manufacture of hydrocracked lubricating oils. It is a further object to provide a method for manufacturing hydrocracked lubricating oils having a low pour point and good resistance to light. It is a further object of this invention to provide an energy-efficient process for manufacturing hydrocracked lubricating oils. These and other objects will become evident to one skilled in the art on reading this entire specification including the appended claims.